Porthole assemblies for submerged illumination of above-ground swimming-pools are known as including generally a lamp unit holder ring and a transparent plate in registry with an opening in the pool wall. Such system is shown and described in Canadian Pat. 1010000 issued to Robert Cantin on May 10, 1977 and assigned to the present applicant. The method of mounting this assembly to the pool wall consists in submerging the cover plate, the securing bolts and the fastening tool while maintaining the holding ring outside the pool in vertical registry with the plate. A further incovenience of present porthole assemblies is that the bolts have portions in contact with the water of the pool causing rust. Also, a gasket is required between the wall and the plate but is often inadequate as water infiltrates through the bolt passages of the assembly.